Scanning MEMS mirror devices comprise a moveable element which comprises a reflective portion. The movable portion is held by one or more holders which hold the moveable portion in a manner which allows the movable portion to oscillate about an oscillation axis. During operation of the scanning MEMS mirror device, the moveable element is oscillated about one or two oscillation axis so that the reflective portion scans light.
The manner in which oscillation of the moveable element is effected varies. Usually it is achieved by means of an electrically conducting coil and a magnet. Either the magnet, or the coil, is position on the movable element, while the other is positioned at a stationary portion of the scanning MEMS mirror device e.g. on the holders. Current is passed through the coil; the magnetic field which is generated by the magnet interacts with the current carrying coil and this forces the movable portion to oscillate about one or two oscillation axis.
Disadvantageously, placing the magnet on the movable element, increases the size and mass of the movable element. Thus, more power is required to oscillate the movable element. Furthermore, the increase in mass will increase the inertia of the moveable portion making it more difficult to oscillate the movable element at a high frequency. If the scanning MEMS mirror device is used in a projector, then oscillating the movable element at a lower frequency will reduce the resolution of the projected image. Furthermore, the size of the magnet is limited by the size movable element; a smaller magnet generates a smaller magnetic field. The smaller the magnetic field the more current which is required to flow in the coil to effect oscillation of the movable element. Thus, the limited size of the magnet which can be provided on the movable element increases the overall power required to effect oscillation of the movable element.
Accordingly, positioning the magnet on the movable element has three main disadvantages; the size of the magnet is limited by the size of the movable element, it increases the mass of the movable element which in turn means that more power is required to oscillate the movable element, and it also increases the inertia of the movable element which makes it more difficult to oscillate the movable element at a high frequency. The provision of a magnet on the movable element of a scanning MEMS mirror is known from DE19803857.
To overcome the above-mentioned problems it is known to provide the magnet on a fixed portion of the scanning MEMS mirror device e.g. on the holders, and to provide the current carrying coil(s) on the moveable element. Such an arrangement is disclosed in WO0218979 and KR20080096090. However the solution proposed in each of these patent applications have disadvantages associated with them.
WO0218979 discloses providing a scanning MEMS mirror device in which has the current carrying coil and a reflective portion are both provided on the same surface of the movable element. As a result the reflective portion does not cover the whole surface of the moveable element. Consequently, only a fraction of the whole area of the surface of the moveable element can be used to reflect and scan light. This reduces the size of the laser spot light which can be reflected and scanned by the scanning MEMS mirror device.
To address this problem it is known to provide the coils on an under-surface of the moveable element. However, due to manufacturing constraints this is very difficult to achieve; in particular, the aspect ratio of an area defined by the holder and moveable element, make it impossible to uniformly and accurately provide coils at the under-surface of the movable element. FIG. 1 provides a cross-sectional view of a scanning 2D MEMS mirror device 10 according to the prior art, in its manufacturing stage (before the device 10 has been provided with a coil). The scanning MEMS mirror device 10 comprises a holder 1 and moveable element 2 (the element 2 can oscillate about two orthogonal oscillation axes; only one oscillation axis 13 is shown). The holder 1 and the movable element 2 define a region 3; the region 3 provides sufficient space to allow the movable element 2 to oscillate about oscillation axis 13. However, region 3 has a large aspect ratio, the large aspect ratio of the region 3 makes it impossible to uniformly deposit material on an under-surface 5 of the moveable element 2. The dashed line 7, shown in FIG. 1, illustrates that a “liquid border effect” results at inner surfaces 9 of the holder 1 if material 11 is deposited on the under-surface 5. The aspect ratio of the region 3 also makes is impossible to uniformly etch any deposited material. Thus, the large aspect ratio of region 3 makes it impossible to deposit material and etch material on under-surface 5 of the moveable element 5 to provide a uniform coil.
The scanning MEMS mirror device provided in KR20080096090 proposes a solution which enables the provision of coils on an under-surface of the moveable element in a scanning MEMS mirror. In the scanning MEMS mirror of KR20080096090 the size of the moveable element is increased so that it matches the size of the holder(s); accordingly the issues associated with the aspect ratio mentioned above are eliminated and the under-surface of the moveable element is now readily available for deposition and etching of material thereon to from a uniform coil. However, as the size of the moveable element matches the size of the holder, when the scanning MEMS mirror device is mounted on a surface, the moveable element will not be able to oscillate. To enable the moveable element to oscillate, additional spacer elements will be required (e.g. applied between the holder(s) and the surface on which the scanning MEMS mirror device is mounted) so as to provide region, between the scanning MEMS mirror device and the surface on which it is mounted, which can receive at least a portion of the moveable element as the movable element oscillates.
It is an aim of the present invention to obviate or mitigate one or more of the aforementioned disadvantages.